CELP speech coders typically use codebooks to store excitation vectors that are intended to excite synthesis filters to produce a synthetic speech signal. For high bit rates these codebooks contain a large variety of excitation vectors to cope with a large spectrum of sound types. However, at low bit rates, for example around 4–7 kbits/s, the number of bits available for the codebook index is limited, which means that the number of vectors to choose from must be reduced. Therefore low bit rate coders will have a codebook structure that is compromise between accuracy and richness. Such coders will give fair speech quality for some types of sound and barely acceptable quality for other types of sound.
In order to solve this problem with low bitrate coders a number of multi-mode solutions have been presented [1–5].
References [1–2] describe variable bitrate coding methods that use dynamic bit allocation; where the type of sound to be encoded controls the number of bits that are used for encoding.
References [3–4] describe constant bitrate coding methods that use several equal size codebooks that are optimized for different sound types. The sound type to be encoded controls which codebook is used.
These prior art coding methods all have the drawback that mode information has to be transferred from encoder to decoder in order for the decoder to use the correct decoding mode. Such mode information, however, requires extra bandwidth.
Reference [5] describes a constant bitrate multi-mode coding method that also uses equal size codebooks. In this case an already determined adaptive codebook gain of the previous subframe is used to switch from one coding mode to another coding mode. Since this parameter is transferred from encoder to decoder anyway, no extra mode information is required. This method, however, is sensitive to bit errors in the gain factor caused by the transfer channel.